I feel like you glossed over tanking in order to talk about your proposed combat scenarios. For tanking you pretty much say "let all melee classes tank." and that's the end of that, then you go on to say, "There shouldn't be zerging because all members of the team should be focusing on their own enemies." In which case, there is no tank. Or are you saying you want each melee character to tank one melee enemy, letting the caster enemies run amok across the entire group? Or are the caster enemies "tanked" by other ranged members of the group? In that case you're not broadening your tank archetype to just melee, EVERYONE is a tank if you're focusing on small scale battles where each character fights one enemy.
Let me play devil's advocate, then: Spellcasters can be interrupted, you say. What's to stop a group from going all melee instead of having any ranged characters? Melee would have an easier time interrupting spellcasters and are able to take more punishment, after all. All you'd have to do to take care of the problem of melee tanking two or more enemies at once would be to zerg the spellcasters, which you don't want to see happen. What happens when I assume you actually have a tank, then? That's when your proposed combat scenarios falls apart, IMO. Tanking involves getting the attention of all enemies onto you instead of your team, therefore negating any "1v1" combat scenarios you were talking about. In that case, you will zerg one enemy at a time in order to defeat them quicker and lessen the overall damage your tank takes.
But if you're saying "let all the melee tank" and want to focus entirely on 1v1 combat, you're kind of taking away the tanking role in the first place, aren't you?
